A subject excretes urine at 2 mL/min with a urine osmolality of 600 mOsm/kg while her plasma osmolality is 300 mOsm/kg. What is her free water clearance (CH2O), and what does it indicate?
- A +2 mL/min, indicating active water diuresis
- B -2 mL/min, indicating that ADH is acting and free water is being returned to the body ✓
- C 0 mL/min, indicating perfect isotonic handling of the filtrate
- D -4 mL/min, indicating maximal medullary concentration beyond the normal limit
Correct answer: B. -2 mL/min, indicating that ADH is acting and free water is being returned to the body
Explanation
CH2O = V x (1 minus Uosm/Posm) = 2 x (1 minus 600/300) = 2 x (-1) = -2 mL/min. A negative value means the urine is hypertonic to plasma, so the kidney is reclaiming solute-free water, which requires ADH action and a functional medullary gradient. Positive CH2O occurs only with dilute urine (Uosm below Posm) during water diuresis with suppressed ADH.
Reference: Ganong's Review of Medical Physiology, 26th ed.
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